The invention relates to apparatus and methods for determining the absolute position of a movable member with respect to one or more fixed members. More specifically, the invention relates to apparatus and methods for determining the absolute position of a manual control for audio systems.
A wide variety of prior art devices and techniques have been developed for determining the absolute position of a movable member with respect to a nonmovable member, or vice versa. Such systems typically rely on a unique physical characteristic which exists with respect to each relative position of the movable and nonmovable members. Such characteristic may be due to resistive, capacitive, inductive, optical, or magnetic properties of the relatively movable members. Systems which are based on one of the above physical properties typically have widely varying characteristics with respect to: i) resolution (the size of the smallest incremental movement which can be detected by the system); ii) repeatability (the error within which a given position can be reproduced by the system); and iii) accuracy (the absolute deviation between the desired target position and the actual position of the relatively movable parts of the system). Furthermore, systems of the above type often vary dramatically with respect to cost, assembly complexity, and susceptibility to ambient conditions which affect resolution, repeatability, and/or accuracy.
Audio systems such as public address systems, audio control panels for mixing audio sources in various venues (e.g., radio studios, theaters, discotheques, etc.) and even home audio systems employ a variety of manual controls in which the output of the manual control is indicative of a particular position of the control with respect to the control panel. In most systems of the type described, the manual control does not itself directly conduct audio power, but rather acts as an indirect control for an amplifier (e.g., a voltage controlled amplifier) which then attenuates or amplifies the audio signal of interest. In this type of application while accuracy and repeatability are important, resolution is more so. Nevertheless, the most important feature for most consumers relates to the lack of electrical noise in the sensor which is otherwise also amplified by the voltage controlled amplifier. For discriminating consumers, the feel of the control (i.e., constant drag with change of position and over time) and durability are also important. Secondary considerations include cost and the absence of xe2x80x9cbleedxe2x80x9d in audio fader controls. xe2x80x9cBleedxe2x80x9d represents the ability of the audio fader control to completely attenuate an audio signal which is controlled by the manual sensor.
Traditionally, potentiometers and variable linear resistors have been used in audio fader control systems because of the relatively low cost of components of this type and large travel distance (particularly with respect to linear resistive control elements) which is highly desirable to audio mixer artists. Conventional wire wound variable resistors having movable wiper arms have been supplanted by the variable resistor of the type having a carbonized resistive element imprinted on a printed circuit board. This mechanical arrangement advantageously facilitates the manufacture of a variable resistor having variable incremental resistance. That is, each incremental movement of the linear fader control produces a non-linear change in the resistor value. This can be achieved by either varying the width or thickness of the resistive trace on the printed circuit board. Such variability is highly desirable because where controls of this type are used for volume control, the human ear""s impression of constant volume change is itself non-linear. Those of ordinary skill in the relevant art are well aware that human perception of increasing volume is not only non-linear, it is essentially logarithmic. This relationship has been quantified by a variety of specialists in the art and is commonly known as a xe2x80x9cstandard listener curvexe2x80x9d. By matching the thickness and/or width of the resistive carbon material on the printed circuit board of a linear variable resistor with a standard listener curve, variability of resistance with respect to position can be produced so as to closely match the standard listener curve. Thus, ordinary amplification circuitry can be employed such that an arithmetic physical displacement of the manual control produces an apparent logarithmic equivalent attenuation in volume of an audio signal which is operatively coupled to that control.
Unfortunately, printed circuit board linear variable resistors of the type described above suffer from a number of defects. First among these defects relates to travel noise associated with movement of the fader control. That is, as the wiper arm traverses the resistive path, microscopic arcing occurs because the travel path itself is not perfectly smooth. Indeed, as the travel path begins to wear, arcing becomes more pronounced, is amplified through the voltage control amplifier, and is heard as a popping or crackling sound by the listener. This problem is only exacerbated by further use and wear. A professional audio mixing artist may cycle an audio fader control up to twenty cycles per second for hours on end. Yet, the life cycle of a typical linear resistive element of the type described may be as little as 10,000 cycles. As the carbonized surface of the printed circuit board begins to wear under the action of the wiper arm, not only is the noise problem exacerbated, but the linear resistor begins to bleed. That is, bringing the wiper to the fully attenuated position associated with its end of travel no longer fully attenuates the signal. A second problem associated with contact type controls such as linear resistors is that the xe2x80x9cfeelxe2x80x9d of the control (i.e., the coefficient of friction of the carbonized surface) is non-constant either over time, or from one end of travel to the other. A good feel which is constant over time is an important characteristic for artists in this field. Finally, the most demanding artists require significant accuracy with respect to controls of this type. However, faders with resistive elements of the type described above typically have an accuracy of no better than xc2x120% total travel length. Thus, even if the repeatability and resolution of the control are good (which typically is not the case), scale markings on the fader control are of little use to the artist due to the low accuracy of the control itself.
Those of ordinary skill in the prior art have recognized the above described limitations of conventional resistive controls, and therefore have developed non-contact position sensors of the capacitive, inductive, optical, and magnetic type. With respect to optical control systems, an optical encoder/decoder system is described by Yochum in U.S. Pat. No. 4,412,812 for use as an audio fader control. In that system, a movable control handle is connected to a linear shutter which is disposed between corresponding pairs of light emitting diodes and light detectors. By appropriately positioning the emitter/detector pairs with respect to various apertures in the shutter, any unique, absolute position of the shutter can be instantaneously determined by digital logic circuitry connected to the light detectors. In addition, the digital information which corresponds to a unique position of the shutter (and thus control handle) can be used to access a digital look-up table in the form of a read-only memory (ROM) which may be provided with a non-linear transfer function, presumably to match the position of the control handle to a standard listener curve. Yochum further describes that more than one ROM can be provided, selected by an appropriate switch, to provide two or more listening curves.
The system described by Yochum avoids all of the disadvantages associated with resistive, contact-type fader control. Nevertheless, Yochum""s system requires at least 13 optical emitter/detector pairs and a mechanical shutter system to provide only 66 unique resolvable positions, although the 8 bit digital logic of the circuitry described therein is capable of storing up to 256 unique data points. Thus, the mechanical constraints of an optical system of this type is a limiting factor in achieving a system with high resolution. In addition, the large number of sensor/detector pairs increases the cost and complexity of manufacturing the device. Finally, the device has a limited range (i.e, linear travel) due to the relatively large physical size of the shutter and emitter/detector pair array. Thus, the effective travel length of the device is inadequate for the most discriminating users.
It has been suggested that magnetic field effects can be used in a digital fader control. Vestax Corporation, Tokyo, Japan, has announced that it will produce such a product.
The desirability of using magnetic sensors for non-contact position information is in itself well known in the art. Skalski has disclosed an elevator control system utilizing non-contact, Hall-effect sensors in a variety of United States patents (e.g., U.S. Pat. Nos. 5,294,757; 5,617,023; and 5,329,077). The Hall-effect is a consequence of the Lorentz force in which a current moving through a sheet of material in the direction of an electric field when subjected to a transverse, steady-state magnetic field results in a voltage appearing in the conductor material transverse to the direction of current flow. In a rectangular sheet of conductive material charges eventually build up on the edges of the sheet which are transverse to the direction of the electrical field, causing a counter-force on the charge carriers moved under the influence of the electrical field. Once the counter-force is in equilibrium with the Lorentz force, a constant voltage will appear at the transverse edges of the conductive sheet, resulting in a transverse electric field. That transverse electric field is measured as the Hall-potential voltage. Thus, as long as there is an electric current flowing through the sheet and a magnetic field applied to the sheet, the Hall-effect voltage will be present whether or not the flux source which generates the magnetic field is in motion or stationary. The magnitude of the Hall-effect voltage is proportional to the cross product of the current flowing through the conductive sheet and the magnetic field applied to the sheet.
Modern Hall-effect sensors have been developed which are relatively inexpensive, have low noise, low offset drift, excellent temperature stability, and very high accuracy, repeatability, and resolution. Thus, Hall-effect sensors have found application in a variety of devices. One example is illustrated by Petersen in U.S. Pat. No. 4,658,214 in which two adjacent Hall-effect sensors are used to detect the displacement of a junction between adjacent north and south poles of two permanent magnets. The position of the magnets is driven by a coil winding, while the magnets themselves are connected to an operative device, such as a camera shutter. Computer controlled circuitry is disclosed which compares a current position of the magnetic junction with respect to a previous position in a feed-back loop to determine whether a motion command has been fully executed. Over a limited range, the flux density in the vicinity of a junction between the north and south magnet poles is substantially linear. This regime is used by the invention described in the ""214 patent. The device disclosed therein is useful for determining displacements of approximately one-half inch. Thereafter, the magnetic flux density is indeterminate with respect to the Hall-effect sensors.
Similarly, the Hall-effect sensors used in the Skalski disclosure are described as being useful to measure displacements on the order of only a few millimeters, whereas audio mixing artists prefer a travel distance for a linear fader control to be on the order of 35 to 40 mm or more.
Thus, a need exists for a non-contact, noiseless, linear fader control for audio applications having long travel length, high accuracy, low cost, and ease of assembly. Such a fader control should also have a long product life cycle and consistent feel over time. The ideal fader control should be impervious to variable ambient conditions and to liquids and sprays which may be applied thereto, and also immune to bleed. Further yet, the desired fader control should have an electrical output well adapted for use by digital signal processing equipment.
It is therefore an object of the present invention to provide an improved fader control for audio applications which is noise-free, has a usable life span of over one million cycles, has a long travel length, has a consistent feel throughout its product life, and is relatively immune to temperature variations, humidity and corrosion.
It is a further object of the invention to provide a fader control which achieves the above objects while being immune to audio bleed and which advantageously uses the natural relationship between flux density and distance to achieve a constant power curve response, also known as a standard listener curve.
It is yet another object of the present invention to achieve the above objects and advantages with a fader control that is part of a system which permits an audio artist to apply an output of the fader control to a digital gain transformation system, permitting the convolution of two data sets, whose mixing is controlled by the audio artist through the output of another manual control to provide a completely custom gain transfer function for the audio mixing artist.
The invention achieves the above objects and advantages, and other objects and advantages which will become apparent from the description which follows, by providing a linear position sensor having a permanent magnet mounted on a lineally-translatable carrier. The carrier is movable between two relatively fixed, spaced-apart magnetic flux detectors, one facing the north pole of the magnet, the other facing the south pole of the magnet. The flux detectors have opposite polarities and are electrically inter-connected such that their combined outputs provide an analog position signal having a magnitude indicative of a specific, monotonic, linear position of the magnet with respect to the flux detectors.
In a preferred embodiment of the invention, the carrier rides on two elongated, laterally spaced apart rails which are parallel to the flux axis of the magnet. The flux detectors are preferably Hall-effect sensors which, when provided with a constant power supply, provide a direct current voltage at the electrical output of each flux detector which is indicative of the position of the magnet and the carrier, even if the carrier and magnet are at rest. The rails are preferably manufactured from a highly polished, durable, non-ferrous material, and the carrier is preferably made from a low friction material to provide an improved feel for the control. The carrier and rails may be provided with a drag-inducing mechanism to properly xe2x80x9cweightxe2x80x9d the carrier. This mechanism may be in the form of a torsion spring.
In the preferred embodiment of the invention, the non-contact fader control of the present invention can be operatively connected to a digital processing circuit which digitizes the analog output from the magnetic flux detectors for subsequent processing, after which the digitized signal is reconverted into an analog signal which controls a conventional voltage controlled amplifier, into which is fed an audio input of choice. The resulting ultimate audio output from the voltage-controlled amplifier is thus a function of the position of the fader control. The digital signal processing circuit can provide a variety of operations on the digitized fader control position information, including low pass filtering, correcting for any initial offset error from the factory, a xe2x80x9chysteresisxe2x80x9d offset function in which any uncertainty in the least significant bit (LSB) of the digital word which represents the fader control position is determined, and xe2x80x9clinearizationxe2x80x9d of the digital signal by normalizing the signal against a xe2x80x9cstandard listener curvexe2x80x9d so as provide a constant power curve response for the ultimate audio output. In addition to the above, the digital processing circuitry can be provided with input from a contour control, in which the contour control mixes data from two different data tables, each table providing a different audio gain transfer function. Thus, the audio artists can customize the ultimate audio output gain curve after the audio transfer function has been adjusted for a variety of factors, including constant apparent power output (i.e., apparent volume).
In an alternate embodiment of the invention, the direct current output of one or more Hall-effect sensors is applied to an analog conditioning circuit which appropriately matches the magnetic properties of the contour control itself such as to mimic the application of a standard listener curve to the fader control sensor output for subsequent application to the voltage controlled amplifier described above.